The present invention relates to a method for producing rocker arms used in a valve-operating system of an internal combustion engine. More particularly, this invention relates to a dimensional improvement of a roller-receiving portion formed of one end of a rocker arm.
A rocker arm is a constituent part for converting rotary motion of a cam which rotates and connects to a valve-operating system, namely a crank shaft in an internal combustion engine, to reciprocative motion of a valve shaft of the intake or exhaust valve of the engine.
In known constructions, since one end of the rocker arm contacts a cam shaft which rotates at high speed, it has been formed with the roller-receiving end portion as an insert made of a wear-resistant, high alloy steel, or by formation of a rocker arm body made of a low alloy steel integral with such end portion. However, in recent years, with high output power and compactness of engines, rocker arms provided with a roller have mainly been employed instead of the described chip.
For instance, a rocker arm is known which has a roller, as shown in FIG. 4. This rocker arm is so formed that it has in its body an opening passing through the upper and lower surfaces for receiving a roller which rotatably contacts with the cam shaft. The described roller is provided in the opening so that the end surfaces of the roller are opposed to the inner surfaces, that is, they come face to face with the inner surfaces of the opening, with a small clearance therebetween.
Furthermore, as shown in FIG. 5, there has also been another type of rocker arm which is of a roller type and is able to operate two valves simultaneously. This rocker arm consists of a rocker arm body and a roller, and is formed with a roller-receiving opening nearly in the central portion of the rocker arm body, the roller being held within the opening. The rocker arm body is divided approximately at its central portion with the sides of the resulting two body portions respectively provided with a pawl contacting a valve. The rocker arm, therefore, is of an approximate U-shape or an approximate Y-shape as seen in a plan view.
Because such rocker arms are rather complicated, they have been manufactured by a sintering method in which a metallic powder is sintered. The sintering method produces a product which is quite accurate in size, but its productivity is limited, which results in an increase of production cost. Otherwise, as other methods for replacing this, a precision casting method and a die forging method have been carried out as a more efficient production method. Although both the precision casting and die forging methods are excellent in productivity, they have low accuracy in size, so that there are seen some problems of being unstable in the space between the valve-contacting, valve-pressing portions and in the space between the opposed inner surfaces of the roller receiving opening in which a roller is held, and being rough on these surfaces.
To solve these problems, for example, Japanese Patent Laid-Open (kokai) No. 91931/1989, of which the entire disclosure is expressly incorporated herein by reference, discloses an improved method for the rocker arm shown in FIG. 4, in which a smooth-faced correction block is inserted into a roller-mounting opening formed in a rocker arm body with a limited clearance therebetween and then an outer surface of the rocker arm body is pressed, said outer surface being opposed to the inner surface of the roller-mounting opening. According to this method, the space between the inner surfaces of the roller-mounting opening is made to a predetermined accuracy and the smoothness of these surfaces is elevated. However, owing to the pressing, the correction block sticks to the inner surfaces of the roller-mounting opening, which results in the drawback that it is hard to dismount the correction block from the opening.
Japanese Patent Laid-Open (kokai) No. 110040/1991, of which the entire disclosure is expressly incorporated herein by reference, discloses another improved method, for the rocker arm shown in FIG. 5, in which a correction block is inserted into a roller-mounting opening of an unfinished product formed to a predetermined shape, and then the unfinished product is pressed at its roller-mounting opening from the outside thereof to make the inner surfaces of the roller-mounting opening smoother as well as to press pawl portions so as to correct the interval between the pawls. According to this method, the space between the inner surfaces of the roller-mounting opening is adjusted to a predetermined accurate size, and in addition thereto the smoothness of these surfaces is improved. However, since the entire breadth of the rocker arm is pressed from the outside thereof, a considerable pressing force is needed, so that residual stresses will occur after deformation and the pressing leads to sticking of the correction block to the inner surfaces of the roller-mounting opening, and therefore there is caused a drawback that the correction block cannot be easily dismounted.
Japanese Patent Laid-Open (kokai) No. 269311/1995, of which the entire disclosure is expressly incorporated herein by reference, discloses a further improved method for the rocker arm shown in FIG. 5, in which a roller-receiving portion is produced in such a way that the space between the opposed sides of the rocker arm is slightly smaller than a predetermined size, and then a pair of correction blocks are inserted between both sides and a press tool is wedged into a gap between the blocks to force the gap open to thereby improve the space between both sides and to improve the smoothness of the side surfaces.
Furthermore, in FIG. 1, a rocker arm is shown which is formed with a roller-receiving portion at one end thereof. In such rocker arm formed with a roller-receiving portion at one end thereof, a problem arises in that the space between the inner surfaces of the roller-receiving portion is apt to be broadened in a heat treatment to a size which exists outside a predetermined accuracy. Thus, an improvement in dimensional accuracy is desired. Also, especially in recent years, small sized rocker arms have been produced by an injection molding technique, in which metallic powder together with a binder is injected and filled into a metal mold in an injection molding machine to form a blank. Such blank formed by an injection molding is heated to remove the binder and thereafter is sintered to form the product. Although the production of parts by injection molding has the advantage of high productivity, it needs sintering at a temperature higher than those in conventional sintering methods, which results in the drawback that a larger change in size will result. In particular, relative to the rocker arm formed with a roller-receiving portion at one end thereof as shown in FIG. 1, the interval between the inner surfaces of the roller-supporting portions of the roller-receiving portion in a sintering and heat treatment process in which a roller is held, is considerably broadened so that there is caused a drawback that a determined accuracy in size cannot be obtained.